Can't Edit Cached Particles
There is a reported bug In Particle Edit mode, Emitter particles are not visible in the viewport. It is about Emitter particles, but Hair particles are not visible either in Blender 2.91.
As a workaround you can

bake/run the simulation
convert the desired frame to a Mesh object in the modifier's panel
use the add-on Hairnet 0.6.3 (for Blender 2.9.1) to generate a new Hair Particle System based on the mesh data

Hairnet will edit the particles and place them along the mesh edges. Use the button 'Fibremesh'. Select the mesh first, then the scalp/head (must be highlighted light orange). The script may run a while depending on the number of particles.